Abstract
Plasma configurations with shear reversal are prone to the excitation of unusual Alfvén eigenmodes by energetic particles. These modes exhibit a quasiperiodic pattern of predominantly upward frequency sweeping (Alfvén cascades) as the safety factor q changes in time. This work presents a theory that employs two complementary mechanisms for establishing Alfvén cascades: (1) a nonstandard adiabatic response of energetic particles with large orbits and (2) toroidal magnetohydrodynamic effects that are second-order in inverse aspect ratio. The developed theory explains the transition from Alfvén cascades to the toroidicity induced Alfvén eigenmodes (TAEs), including modifications of the TAEs themselves near the shear reversal point.
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